1. Field of the Invention
This invention relates to apparatus for manufacturing webs of synthetic thermoplastics, comprising a machine frame, a cooling roller mounted in said machine frame and provided with drive means, and an extrusion die mounted in said machine frame and spaced above said cooling roller and connected to a supply line for molten plastic material and having a die slot which faces the cooling roller and is parallel to a generatrix of said roller, also comprising two electrodes, which in the direction of rotation of the cooling roller precede the slot and are disposed adjacent to the lateral end portions of the slot and mounted in the machine frame and directed toward the cooling roller, and electric cables electrically connected to the electrodes and to the cooling roller.
2. Description of the Prior Art
When a flat film of molten plastic material is extruded through a slot die onto a cooling roller, which is disposed below the slot die and rotates at a surface velocity which is equal to the extrusion velocity, that film will laterally contract on the cooling roller with formation of thickened marginal regions. That undesired phenomenon is described as "neck-in effect".
When the film of plastic material is extruded at a velocity of 60 to 70 m/min, the film which has just been extruded can be urged against the cooling roller by compressed air so that the neck-in effect will be avoided.
For an extrusion at higher velocities, high-voltage electrodes of the kind described first hereinbefore are used, which are known from U.S. Pat. No. Re. 27,771 and by which the thermoplastic film which has just been extruded is electrostatically charged in such a manner that it will subsequently adhere to the surface of the cooling roller and an entrapping of air by the film and the undesired necking of the film being cooled will be substantially avoided. The high-voltage electrodes used in the apparatus known from U.S. Pat. No. Re. 27,771 consists of wire electrodes, point electrodes or electrodes which extend like blades across the width of the extruded films. But it has been observed in practice that by means of such electrodes the freshly extruded film cannot be secured to the cooling roller so firmly that the undesired neck-in effect will be avoided.